1. Field of the Invention
The present invention relates to thermoplastic polymer foam having nanometer sized cells and a process for making such foam.
2. Introduction
Polymeric foam articles (or simply “polymeric foams”) are common in thermal insulation applications. Many characteristics of polymeric foam affect the thermal conductivity through the foam and, hence, the effectiveness of the foam as a thermal insulator. For instance, it is known that heat transfer through polymeric foam insulation can occur by conduction, radiation and convection (see, for example, teachings in United States patent application publication 2009/0148665). In typical polymeric foam insulation the dominant mode of heat transfer is cell gas conduction, which contributes approximately 75% of the total thermal conductivity. Hence, reducing conduction of cell gas can significantly reduce heat transfer through polymeric foams.
One characteristic affecting thermal conductivity contribution of cell gas is cell size. Cell size has little influence on gas thermal conduction when the cell size is between about one micron and one millimeter in size. Above one millimeter convection behavior tends to increase thermal conductivity. However, when the cell size of foam is less than about one micron the gas conductivity decreases due to what is known as the Knudsen Effect (see, for example, the relationship illustrated in FIG. 1. The curve follows the methodology in Lee, et al., “Determination of a mesopore size of aerogels from thermal conductivity measurement”, Journal of Non-Crystalline Solids, March 2002, Vol. 298, pages 287-292). The Knudsen Effect is a phenomenon that results in a decrease in thermal conductivity as fewer cell gas molecules are available within each cell to collide and transfer heat within each single cell. The Knudsen Effect becomes significant as the cell size and connectivity between cells becomes on the same order of magnitude as the mean free path of the gas filling the cells. Thermal conductivity due to cell gas reduces almost in half when the cell size reduces from one micron to 300 nanometer (nm), and reduces by almost ⅔ when the cell size reduces from one micron to below 100 nm. Therefore, it is desirable to achieve cell sizes of 300 nm or less, preferably 100 nm or less to minimize thermal conductivity through foam.
It is further desirable to achieve a homogeneous cell size distribution. Even occasional large cells can reduce the thermal insulation effect of the small (300 nm or less, preferably 200 nm or less, still more preferably 150 nm or less) cells. Therefore, all things being equal, reducing the average cell size of foam to 300 nm or less and particularly to 200 nm or less is desirable to achieve lower thermal conductivity through the foam, especially in foam having a homogeneous cell size distribution. However, it is difficult to reduce the cell size without affecting other properties of a polymeric foam article.
Porosity, the ratio of void volume to foam volume, also affects the thermal conductivity of polymeric foam. Porosity can be expressed as a ratio with a value less than one or as a percentage, which is the ratio multiplied by 100. Generally, decreasing porosity results in an increase in thermal conductivity. That is because thermal conductivity through the polymer network that makes up the walls defining cells of foam is typically greater than thermal conductivity across gas within the cells.
Polymeric foam having an average cell size of 300 nm or less and a porosity of greater than 0.50 is highly desirable but difficult to achieve with known blown foam technology heretofore. Notably, blown foam technology is desirable because unlike aerogel technology, for instance, blown foam technology does not require large volumes of solvents to manufacture.
In developing a process for producing foam having a particular cell size it is useful to consider the number of effective nucleation sites. Effective nucleation sites are the number of sites in a foamable polymer composition that form voids, or cells, when the foamable polymer composition expands into foam (also known as “cell density” in, for example, a paper entitled “A Process for Making Microcellular Thermoplastic Parts” by Kumar and Suh, Polymer Engineering and Science, October 1990, Vol. 30 No. 20, pages 1323-1329). By controlling the number of effective nucleation sites and the porosity one controls the average cell size of the foam. In order to achieve a desirable thermally insulating foam it is desirable to prepare polymeric foam having at least 3×1014, and preferably 1×1015 or more, effective nucleation sites per cubic centimeter of non-foamed polymer matrix material (nucleation density) and expand that to have a porosity that is greater than 0.70 (or 70% when as expressed as a percentage). It can be a challenge to induce the necessary number of nucleation sites and achieve the stated porosity when preparing foam having nanometer sized cells.
It would be a desirable advancement in the art of thermally insulating polymer foam to be able to prepare blown polymeric foam having a thickness of at least one millimeter and having a nucleation density of at least 3×1014, preferably at least 1×1015 effective nucleation sites per cubic centimeter of non-foamed polymer matrix material and that has expanded to have a porosity that is greater than 70%. Even more desirable would be such polymeric foam that has an average cell size of 300 nm or less, preferably 200 nm or less, more preferably 150 nm or less and yet more preferably 100 nm or less.
Polymeric foam achieving at least part of these desired features has been developed containing nanometer-sized filler particles (nanofiller) as reported in published patent application WO 2011/066060. However, such additives can increase viscosity of a polymer composition as their concentration in the polymer composition increases. As a result, there is a practical limit to the amount of nucleating particles that can be added and efficiently dispersed into a polymer composition for foaming. It is desirable to be able to prepare such a foam without requiring a nanometer-sized filler (that is, in an absence of nanofillers).
Polymeric foam achieving at least part of the aforementioned desired features has also been developed without requiring a nanofillers provided a particular thermoplastic polymer is present as the primary component in the thermoplastic polymer matrix, as reported in WO 2011/112352. Despite the advancements reported in WO 2011/112352, it is yet desirable to find a way to achieve a porosity that is greater than 70%, while further increasing the nucleation density and/or achieving smaller cell sizes over the particular polymer technology disclosed in WO 2011/112352. This is particularly desirable when using a carbon dioxide blowing agent.
Other nanofoam art further illustrates room for advancement in the art field.
Ruiz et al., in Journal of Supercritical Fluids 57(2011) 87-94, disclose a two-step method for making microcellular foam having a cell size in a range of 0.3-300 micrometers using a polymer composition that requires a triblock copolymer. It would advance the art to have a process for preparing nanofoam without requiring a triblock copolymer
US2009/0130420 provides a method for preparing polycarbonate nanofoam provided the polymer comprising structural units derived from 2-hycrocarbyl-3,3-bis(hydroxyphenyl)phthalimidine compounds. It is desirable not to have to use this specific polymer or be limited to polycarbonate technology.
Foaming processes utilizing extremely high pressures of carbon dioxide (see for example, U.S. Pat. No. 6,555,589 and U.S. Pat. No. 6,555,590) or explosive depressurization at rates of 15,000 to 200,000 MegaPascals per second (see for example US20110287260) have also been taught as useful to produce polymeric nanofoam. However, the engineering requirements to achieve the required high pressures of carbon dioxide and/or explosive pressure release rates are too extreme to be of practical interest in producing large foam specimens.
It would provide an advance in the art of nanofoam technology to identify a process for preparing thermoplastic polymer nanofoam that has a porosity that is greater than 70%, while further increasing the nucleation density and/or achieving smaller cell sizes over the technology disclosed in WO 2011/112352 and without requiring a triblock copolymer, nanofillers, special polycarbonate polymers or extreme depressurization rates.